The present invention relates to a method for stabilization of a transparent photonic network having at least one wavelength division multiplexing (WDM) path which has a specific number of channels on which signals can be transmitted, and to a method for reconfiguration of a transparent photonic network from a first WDM path to a second WDM path between two points which have a number of channels on which signals can be transmitted.
The rapid growth in the Internet has resulted in an equivalent increase in the amount of data traffic. The operators use WDM systems in order to allow appropriate transmission capacities to be provided. The systems are generally operated as purely static point-to-point systems, passing the entire data stream from a point A to a point B via the same transmission path over a relatively long time period. However, the operators of optical networks would like to increase the flexibility by being able to dynamically match the optical networks to changes in the amount of traffic. This is achieved by using a transparent network composed of intermeshed WDM paths. At the nodes, there are transparent switching matrices, which allow the data streams to be switched flexibly on the basis of individual wavelengths. This is known as dynamic wavelength routing. The consequence of such a method is that the WDM paths are operated with a continuously changing number of channels. The frequent connection and disconnection of channels regularly leads to a deterioration in the signal quality of the remaining channels owing to nonlinear effects in the optical amplifiers, or in the transmission fiber.
The WDM paths used at the moment operate with such high total levels at the input of the transmission fiber that the optical amplifiers need to be operated in their saturation region. Linear operation of the amplifiers would necessitate a different design with considerably higher pump power levels, which would lead to very inefficient use of the pump power, and to higher costs. Saturation operations occur when the gain for the individual channels depends on the input power and on the number of active channels, without any adaptation of the pump power. Without countermeasures, connection or disconnection of channels results in disturbing level fluctuations in the remaining channels. In order to counteract this effect, the gain of the optical amplifiers is controlled. This gain control, for example, measures the total level both at the input and at the output of the amplifier, and keeps the ratio between the two levels constant by matching the pump power. If the amplifiers have a flat gain spectrum and the gain control operates sufficiently quickly, this can ensure that the gain is constant in the remaining channels. Level fluctuations in the remaining channels, caused by amplifier saturation, when channels are connected and disconnected can be suppressed adequately via such gain control.
However, owing to the major growth in transmission capacity, an immense increase in the number of channels is required, which is leading to ever broader transmission bands and increasing total power levels being used at the input to the transmission fibers. In a system which, for example, is operating in the wavelength band from 1,530 to 1,565 nm (C band) and in a wavelength band from 1,570 to 1,605 nm (L band), 160 channels are transmitted in each band, with a data rate of 10 Gbps per channel. The total power levels at the fiber input are in this case greater than 23 dBm. Such widely used wavelength bands and high input power levels result in the channel level distribution in the transmission fiber being distorted by stimulated Raman scatter (SRS). The extent of distortion in this case depends on the input power level, and hence on the number of active channels. When channels are connected and disconnected, the distortion varies with time constants on the order of milliseconds or less. The remaining channels are thus subject to rapid level fluctuations in each path section. When gain control is used for the optical amplifiers, level fluctuations in each path section are additive, resulting in large level fluctuations on the order of several dB at the end of the path. Unless countermeasures are taken, these fluctuations can lead to transmission faults, to failure of individual channels, or even to failure of the entire path.
The distortion of the channel level distribution in the transmission fiber resulting from SRS can be compensated for statically via variable attenuators or filters in the intermediate amplifiers. However, when channels are connected or disconnected, the compensation mechanism must be readjusted in an appropriate manner. Complete avoidance of level fluctuations produced by SRS with the short time constants referred to is impossible to achieve, or can be achieved only with an extremely high level of complexity.
An object of the present invention is, therefore, to provide a method for stabilization and reconfiguration of a transparent photonic network, whereby level fluctuations can be avoided, despite short time constants.